The present invention relates to medical radiography, and more particularly to means for identifying an individual X-ray cassette from among a group of similar cassettes within a given x-ray department.
2. Description of the Prior Art
A radiographic cassette is generally a flat box of rigid construction designed to contain a sheet of x-ray film in a light-proof condition during the time it is to be exposed to x-ray for the purpose of creating a medical radiograph. From time to time the inside of the cassette may become contaminated with a foreign object, dirt particle, ink smudge, etc. This foreign object makes its presence known by causing an undesirable flaw to appear on radiographic films subsequently exposed in that cassette and one normally attempts to locate and clean the defective cassette. However, the defect is situated on the interior of the cassette and most cassettes are normally maintained in their ready condition, loaded with film and sealed against light. To find the contaminated cassette, all cassettes of the same size in the department would have to be opened, unloaded, and inspected. To solve this problem, it has been customary to write identifying indicia on one of the intensifying screens within eadh cassette and record the same indicia somewhere on the cassette exterior. If, for example, one can determine that a defective film bears the identifying indicium "A8A", one would search among all cassettes in that department for the one with the same indicium "A8A", written on the outside, selectively open only that cassette, clean its screens, and return it to service.
In practice, such attempts to correlate flawed film with its source cassette are unsuccessful when the indicia identifying the intensifying saeen are not visible on the corresponding radiograph. This lack of visibility generally results from the fact that such identifying indicia heretofore have been written or mounted to one intensifying screen only and are therefore overexposed or "burned out" in the upper range of exposures often required for medical radiography. Frequently the image of the indicia is not visible on the resulting radiograph. The dynamic range of clinically useful x-ray exposures is about 2000, i.e. the exposure required for a satisfactory radiograph of a large body part may be 2000 times greater than that needed for a small part. Each intensifying screen within the cassette contributes approximately half of the film exposure. As exposure is increased, light emanating from an umnarked screen opposite the identifying indicium is often times sufficient to darken the film by itself and render the image of the identifying indicia invisible. More times than not, indicium placed on one screen only is visible when the radiograph receives a relatively low radiation exposure but becomes progressively invisible as the exposure is increased. In the latter circumstance, isolating a cassette with a flaw is a difficult and tedious task.
Apparatus and methods, including modifications of the film cassette for marking exposed X ray film or radiographs are disclosed, for example by U.S. Pat. Nos. 3,628,864, 3,703,272, 4,465,364, 4,510,392, 4,806,959, 4,383,329, 4,520,497, and 4,768,114. These art teachings do not address the problems presented by overexposure of identifying indicia. Further, no method has been previously described for mounting indicia to both intensifying screens, in opposition to one another, thereby producing a summation image which is virtually immune from such overexposure. Thus remains a need in the art for a method and means for readily differentiating flawed cassettes from numerous cassette candidates in an accurate, reliable manner.